1. Field of the Invention
The present invention relates generally to buffer amplifiers.
2. Description of the Related Art
The principal purpose of buffer amplifiers is to provide isolation while coupling signals between a preceding electronic circuit and a succeeding electronic circuit. Accordingly, a perfect buffer amplifier presents an infinite impedance to the preceding circuit and has zero output impedance and infinite bandwidth to facilitate driving the succeeding circuit. If the preceding circuit provides an input voltage Vin, the perfect buffer amplifier supplies the current required to drive the succeeding circuit and provides an output voltage Vout equal (although possibly offset) to the input voltage Vin.
Buffer amplifiers are essential elements in a great variety of signal-conditioning systems (e.g., analog-to-digital converters). Because real buffer amplifiers fail to provide the ideal parameters described above, their transfer functions generally exhibit nonlinearity so that they insert distortion into the output voltage Vout and thus degrade the performance of these systems.
Although this distortion can be reduced by increasing transistor bias currents to greatly exceed signal currents (thus reducing the relative effect of the signal current variations), this modification significantly degrades the buffer amplifier""s efficiency. If the buffer amplifier""s output load is capacitive, linearity will still degrade with increased signal frequency because of the increased current demand that is required to drive the load capacitance at higher frequencies.
The present invention is directed to buffer amplifiers that realize enhanced linearity.
These goals are realized with a replica current generator that supplements a buffer transistor and is configured to provide a replica current which substantially equals required load currents in the amplifier""s output load. Because the current of the buffer transistor 24 now remains constant, its base-emitter voltage Vbe remains constant and the amplifier linearly reproduces the input signal Sin across the output load.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.